1. Field of the Invention
The present invention generally relates to amplifying signals, and more particularly to a variable-gain amplifier and a method for controlling the same.
2. Background of the Related Art
Gain-controlled amplifiers are used in many wireless and wireline systems. In wireless applications, gain-controlled amplifiers which demonstrate a linear characteristic throughout a desired operational range are especially important for purposes of achieving a satisfactory level of performance.
FIG. 1 shows one wireless application in the form of a cellular communications receiver, which includes variable-gain amplifiers along its signal path. The receiver includes an antenna 1, an RF bandpass filter 2, and a low-noise amplifier 3. The signal output from the low-noise amplifier is combined, in mixers 4 and 5, with phase-shifted versions of an oscillator signal. In this receiver, the oscillator signal is set to the carrier frequency and thus baseband-signal recovery is performed using one conversion. (A receiver of this type is often referred to as a direct-conversion receiver.) The output of each mixer is passed through a low-pass filter LPF, amplified by a variable-gain amplifier VGA, and converted into a digital signal by an ADC converter. Subsequent signal processing steps are then performed.
In direct-conversion and other types of receivers, variable-gain amplifiers are used to suppress noise introduced into the baseband signal along the receiver signal path. The amount of noise suppression that takes place is typically proportional to the gain of the amplifier. When the received signal level is smaller than desired, the gain of the amplifier is increased. Conversely, when the received signal level is higher than desired, the gain of the amplifier is decreased. By adjusting the gain of the amplifier and accordingly the level of the baseband signal, excessive constraints on the dynamic range of subsequent stages of the receiver (including the analog-to-digital converter) can be avoided.
In wireless applications implemented using variable-gain amplifiers, increasing amplifier linearity is considered important for obtaining an acceptable signal-to-noise ratio. Unfortunately, when these amplifiers lack sufficient linearity, the desired signal is corrupted by inter-modulation caused by strong interfering signals.
FIG. 2 has two signal diagrams which show, by comparison, one way in which interference can affect the signals in a wireless application. The first signal diagram shows the state of a signal in a communications receiver such as shown in FIG. 1 prior to being input into one of the variable-gain amplifiers. This signal includes a desired signal and multiple interfering signals which have substantially the same amplitudes propagating along the signal path. When the desired and interfering signals have small magnitudes, the desired signal can be amplified to a desired level in spite of any linearities that may exist in the VGA. This is shown in the second signal diagram, where the amplitude of the desired signal is greater than the amplitudes of interfering terms generated by inter-modulation with the interfering signals. (In FIG. 2, Psig represents baseband signal power, which is shown to be triangular in shape and where the height of the triangle is directly proportional to the power).
FIG. 3 has two signal diagrams which show, by comparison, another way in which interference can affect the signals in a wireless application. The first signal diagram shows the state of a signal in a communications receiver such as shown in FIG. 1 prior to being input into one of the variable-gain amplifiers. However, unlike FIG. 2, the interfering signals have significantly larger amplitudes than the desired signal. Consequently, when amplified by a variable-gain amplifier having non-linear characteristics, the desired signal is seriously corrupted by noise terms generated from inter-modulation with the interfering signals. This is shown in the second diagram, where the amplitudes of the noise terms are much greater than the desired signal amplitude. If left uncompensated, this noise will propagate throughout the receiver to degrade the quality of the received signal.
FIG. 4 shows a variable-gain amplifier which has been proposed for use in a receiver. The amplifier is formed from a single differential amplifier 50 which includes two feedback paths 51 and 52, four resistors, and a virtual ground provided at the amplifier inputs. Resistors R1 are placed at the inverting and non-inverting terminals and resistors R2 are located along the feedback paths. The values of resistors R1 and R2 control the gain of the amplifier, i.e., changing the values of variable resistors R1 and R2 will result in setting the amplifier to a desired gain as indicated by the following equation:
                                          V            OUT                    -                      V            OUTB                          =                                            R              2                                      R              1                                ⁢                      (                                          V                IN                            -                              V                INB                                      )                                              (        1        )            
where Vout and VoutB are the differential output voltages of the amplifier, Vin and VinB are the differential input voltages, and the ratio of R2 and R1 defines the gain.
Equation (1) defines the gain of the amplifier under ideal operational characteristics. In practice, however, the gain is not infinite and the amplifier suffers from secondary effects. For example, because the amplifier gain is not infinite, the input nodes of the amplifier will slightly track the input signal. The amount of fluctuation that occurs at the inputs depends on the gain and frequency characteristics of the amplifier.
FIG. 5 shows a block diagram of the operational trans-conductance amplifier of FIG. 4. As shown, this amplifier may be modeled using five transistors, where the gates of transistors M1 and M2 receive respective differential inputs IN and INB, the gates of transistors M3 and M4 receive a control signal from common mode feedback circuitry (CMFB), and transistor M5 is provided to set the bias current of the operational amplifier from the external bias circuitry not shown in FIG. 5. The common mode feedback circuit is used to stabilize the common mode output voltage of the two output signals, OUT and OUTB. Nodes N1 and N2 respectively disposed between transistor pair M1 and M3 and transistor pair M2 and M4 provide the differential output voltages OUT and OUTB of the amplifier. These voltages are fed back to the CMFB, where they are used to set the common mode output voltage of the two output signals. M1, M2, and M5 are NMOS transistors, M3 and M4 are PMOS transistors, and VDD is a supply voltage connected to the sources of transistors M3 and M4.
The non-linear properties of the amplifier are mostly attributable to the common source node (A) of the input transistors. More specifically, since the amplifier is usually designed to have very high gain at its input stage, small distortion at the input stage generates large distortion at the output stage. This large distortion results from the transient behavior node A experiences as a result of the two current signals flowing in the opposite directions (this opposing flow is explained in greater detail below). As a result, harmonics are generated at node A which alter the linear characteristics of the amplifier and thus generate the large distortion that occurs at the amplified output. The currents signals may be explained in greater detail as follows.
Since the two input signals, IN and INB, operate as a differential signal from the centered common mode signal, the two inputs signals can be expressed as follows:
            V      IN        =                  V        CM            +                        V          D                2                        V      INB        =                  V        CM            -                        V          D                2            
where VCM is defined by the common mode feedback circuit. Thus, when the input voltage increases, the voltage of INB decreases. With this condition, the relative change of the current flowing into M1 and M2 transistors will have different polarity; that is, when the current in M1 increases the current in M2 decrease, and when the current in M1 decreases the current in M2 increases. The two current signals into the M1 and M2 transistors may therefore be said to flowing in opposite directions.
Another source of non-linearity in the amplifier of FIG. 5 is the common mode feedback circuitry. While this circuitry is beneficial for purposes of stabilizing the output levels of the amplifier, it produces mixed harmonics which cause distortions in the output signal.